Microemulsions are a broad class of micro- and nano-sized fluid particles. They can achieve very low surface tension, can simultaneously have oil, water, surfactant phases, and can be designed to transport a wide variety of chemicals in different thermodynamically stable phases. Specifically, in some supercritical fluids like CO2 used for Enhanced Oil Recover (EOR) projects in the oil and gas industry, the addition of certain microemulsions enhance the supercritical fluids' ability to dissolve highly polar, ionic, high molecular weight species. These high molecular weight species are often found in oil and gas EOR reservoirs.
A microemulsion is a mixture of water, water insoluble and water soluble components forming a visually homogeneous, transparent liquid. One or more active ingredients may be present in the aqueous phase, the non-aqueous phase, or in both phases. A variety of microemulsion formulations may be prepared in which the aqueous phase can be considered the dispersed phase, the continuous phase or, alternatively, where the two phases are considered to be bi-continuous. In all cases microemulsions will disperse into water to form either conventional emulsions or dilute microemulsions. Microemulsion solutions form a unique class of emulsions normally consisting of an aqueous phase, and oil phase and a surfactant phase. This class of emulsion is very small, in the nano- to micro-meter range, is usually optically clear and thermodynamically stable, as opposed to other emulsions that are kinetically stable; and microemulsions normally have low viscosities and ultra low interfacial tension properties. The formation of a microemulsion requires the appropriate blending of an oil phase, a water phase, and at least one surfactant. It is often necessary to add salts to enhance the thermodynamic stability of the emulsion depending on the specific blend of the solution. Non-limiting examples of compositional blends that can be used to form a microemulsion include solutions consisting of 13%-55% of fluids from the turpene group (examples include limonene and others), 0%-30% isopropanol, 0-50% water, 0%-50% triethylene glycol, and 0%-15% salts.
The fact that these are very small particles with reduced surface tension allows them to get into low permeability and low porosity subterranean reservoirs. When injecting fluids into subterranean reservoirs during secondary and tertiary recovery to sweep out the hydrocarbons it is useful to be able to get the injection fluid into all areas if the reservoir, including the low permeability and porosity areas of the subterranean reservoir as the higher permeability and porosity structures are most easily recovered and likely have their hydrocarbons significantly or substantially exhausted during the primary recovery phase of the reservoir.
Presently, the use of microemulsions in oil and gas production has been limited to fracturing and acid stimulation operations. Fracturing operations involve the pumping of hydraulic fluids at high pressure (i.e., pressures above the hydraulic fracturing pressure of the reservoir) into the subterranean reservoir formations of the subterranean zone to crack the subterranean reservoir and enhance the subterranean reservoir permeability. This causes hydraulic fracturing of the subterranean formations, and the release of hydrocarbons through the resulting enhanced permeability, thereby improving hydrocarbon recovery. Microemulsions then allow for and enhance the recovery of fracturing fluids used in the operation to be produced back out of the production well reducing fluid damage in the reservoir and thereby increasing the ability of a well to produce from the fracture system made by these fluids.
Blends of microemulsions and supercritical fluids have not been used for the recovery of oil and gas outside of the context of fracturing and stimulation injection operations. For example they have not been used in secondary and tertiary hydrocarbon recovery where supercritical fluids are often used. In fracturing and stimulation operations, the microemulsion is blended in a fracturing or stimulation fluid at low concentrations, typically on the order of 0.2% of the blend. Additionally, microemulsions have not been used as the actual fracturing or stimulation fluid, but only as an additive to a fracturing fluid. Furthermore, the separation and recycling of the microemulsion from a fracture or stimulation fluid has never been performed. Additionally, fluid compositions comprising microemulsions and supercritical fluids have not been used for the enhanced recovery of oil and gas. The present invention permits enhanced recovery of hydrocarbons using microemulsions with supercritical fluids and further provides a method to reduce the cost of the microemulsion application by providing for the separation and recycling of the microemulsion once it is produced back to surface with the well fluids.